All-terrain vehicles are relatively lightweight with a relatively low center of gravity. Early three wheel versions had knobbly tires having small square block elements and a relatively shallow tread depth.
Later versions of “Quad runners” or 4-wheel type ATV's were developed and have been more widely accepted due to their improved stability. Horsepower increases and improvements in both vehicle suspension and chassis has resulted in vehicles capable of relatively high speed and much greater load carrying capacity.
The tires used on these vehicles are operated at very low pressures in the 0.7 bar (10 psi) or less range. The tires are very wide with relatively large air chambers which assist in absorbing shock and vibration. The tires generally have nominal rim diameters of 36 cm (14 inches) or less and overall diameters of 69 cm (27 inches) or less.
Often the rear tires are of a slightly larger size than the more lightly loaded front tires.
In aggressive off-the-road applications, the tires must have a very open tread pattern that employs elongated lugs which provide effective straightline or drawbar traction to enable the vehicle to climb hilly and rough terrain, as shown in U.S. Design Pat. No. 308,038. Additionally, the tread must provide excellent lateral traction for vehicle stability during turning maneuvers as is disclosed in U.S. Pat. No. 5,259,429.
The tire disclosed in that patent employs a repeating pattern of long, intermediate length and short lugs arranged to provide improved traction. The arrangement of these lugs is such that each lug wraps completely over the tread shoulder portion. The lugs are also circumferentially relatively closely spaced such that numerous lugs are in the footprint of the tire at any one time. The tire made according to this prior art invention is considered one of the best mud tires in its class according to its manufacturer.
In wet soils with heavy clay content there is a tendency for the tread to pack with mud between the elongated lugs. The circumferential space between the adjacent lugs is commonly referred to as a soil discharge channel. The channels extend generally from the centerplane of the tread axially outwardly over the tread shoulder. Once this area is packed with mud, the tread effectively looses its ability to provide any traction. This is because the lugs are buried in the packed mud giving the tire the appearance of a slick or racing tread devoid of grooves.
An objective of the present invention is to provide a tread pattern which is self-cleaning in wet clay soil conditions.
It is a further object that the tread design have good handling and traction performance on firm soils.